The present invention relates to a surveying instrument with a rotating unit, e.g. a surveying instrument with a tracking function and with a distance measuring optical system accommodated in the rotating unit. The invention also relates to a surveying compensation method on this type of surveying instrument.
A surveying instrument with the tracking function performs surveying of a measuring point by tracking an object to be measured such as a reflection mirror. By this type of surveying instrument, a surveying operation can be carried out even when a surveying operator is not assigned at a side where the surveying instrument is installed.
In this type of surveying instrument, an optical axis of a distance measuring optical system (i.e. a distance measuring light optical axis) is rotated in a horizontal direction and the optical axis of the distance measuring optical system can be rotated in a vertical (high-low) direction, so that the optical axis of the distance measuring optical system is always directed toward the object to be measured. An horizontal angle and an elevation angle of the distance measuring light optical axis are detected by an angle detector. A distance to the object to be measured is measured, and a horizontal angle and an elevation angle of the object to be measured is measured.
In general, in order that a rotating unit can be rotated, there must be a gap between a rotation shaft and a bearing unit. The rotating unit has an eccentricity, an unsteadiness of rotation, etc. caused by error in the manufacturing process. Because of the gap, the eccentricity, the unsteadiness of rotation, etc., a deviation occurs on the distance measuring light optical axis during rotation. Such type of deviation on the distance measuring light optical axis appears as an error of a horizontal angle or an elevation angle. For the purpose of improving the measurement accuracy, it is necessary to eliminate the influence of the deviation on the distance measuring light optical axis.
In addition to the error caused by the rotating unit, there is a case where an error is caused by the angle detector itself. In order to perform measurement with high accuracy, it is necessary to compensate the error of the angle detector.
A surveying instrument, which is capable to compensate the deviation on the distance measuring light optical axis during the rotation is disclosed in JP-A-2003-185436.
According to the Patent Publication of JP-A-2003-185436, an encoder plate (a disk where a pattern for an angle detection is formed) rotated in a horizontal direction is used as a reflection mirror. A first detecting light is projected to the encoder plate, and the first detecting light reflected from the encoder plate is received at a photodetector. Then, a second detecting light is projected to a free liquid surface, and the second detecting light reflected from the free liquid surface is received by the same photodetector. From a deviation of a photodetecting position on the photodetector, a tilting of the encoder plate with respect to the free liquid surface, i.e. a deviation of the distance measuring light optical axis, is detected, and a measured value is compensated based on a result of detection.
The invention disclosed in JP-A-2003-185436 is based on an assumption that the encoder plate is mounted in a perpendicular direction with respect to a rotation shaft, and that an undulation or the like does not exist on the encoder plate itself, or on the reflection surface. In fact, however, the encoder plate may not be in the perpendicular direction with respect to the rotation shaft due to the error at the time of mounting, or the undulation or the like may be present on the reflection surface. In such case, a deviation of the photodetecting position on the photodetector does not accurately reflect a deviation of the distance measuring light optical axis. In such case, the compensation cannot be performed with high accuracy.